This invention relates to a spectral monitoring device for use in combination with a layer processing device in which a layer is processed by the use of plasma. The processing may be either of etching and forming, as by sputtering or vapor deposition.
Monitoring devices of various types have been used in monitoring the status of a layer being processed in a chamber of a plasma processing device. Examples are a spectral monitoring device, a crystal thickness monitoring device, and an atomic absorption monitoring device.
Monitoring devices of two latter types are for use in monitoring the rate of growth of the layer. Such a device is incapable of indicating the precise rate of growth. It takes an appreciable time in obtaining the result of monitor. The device is therefore unsuitable on strictly controlling the thickness of the layer being formed. In addition, it has been impossible to use the device in monitoring a layer being etched.
Spectral monitoring devices have been used in monitoring a layer being etched. The devices are operable on various principles.
With conventional spectral monitoring devices, completion of etching is monitored by detecting disappearance of the spectrum of the layer being etched from the light beam resulting from the plasma. The device is advantageous in that the structure is simple. The spectrum being monitored, however, varies also when fluctuation takes place in the source voltage of the high-frequency power surce for use in generating the plasma. Such fluctuation is inevitable because the pressure and the composition of the gas in which the plasma is generated, are variable with the progress of etch. It is therefore difficult to avoid excessive etching of the layer. Furthermore, the device must have a high spectral resolution because spurious spectra may appear adjacent to the spectrum of the layer.
With a spectral monitoring device disclosed in Japanese Unexamined Patent Publication No. Syo 53-50794, namely, No. 50794/78, assigned to the present assignee, completion of etching is detected by calculating a difference between intensities of two beam components included in different spectrum regions. The completion of etching is indicated when the difference is kept constant as regards time. This system is effective to remove the influence of the above-mentioned fluctuation of the source voltage. However, it is not really easy to determine whether the difference becomes constant or not. Therefore, an intricate circuit, such as a microcomputer, is necessary to detect the completion of etching. In addition, the system is hopeless of precise detection because the variations of both spectra tend to be cancelled relative to each other by calculation of the difference. The spectra are derived through different windows attached to the chamber of the etching device. Therefore, each spectrum is subjected to an influence of contamination of each of the windows and is individually changed in its intensity.